1. Field of the Invention
This invention relates to an electrophotographic process which makes it possible to provide copied images from positive to positive (or negative to negative) and positive to negative (or negative to positive) in relation to the original simply by changing the polarity of charging through the use of a photosensitive element having an electrically insulating surface layer.
2. Description of the Prior Art
Of photosensitive elements hertofore practically used for electrophotography, typical elements are those in which layers of a photoconductive substance such as Se, Se alloy, Zn oxide and cadmium sulfide are provided on an electrically conductive substrate. However, all of these photosensitve elements have been usable only where the charging has either a positive or negative polarity. The foregoing photosensitive elements cannot be used for charging of polarities because the charging is poorer for one polarity, the sensitivity thereof is deteriorated due to a poorer movement of electric charge carriers for one polarity, or because a space charge trapped in the bulk is accumulated in the photosensitive element and thus, the residual potential increases. In the case of an Se system photosensitive material, for instance, the following problems have occurred. When the material is negatively charged, positive charge carriers are injected from the electrically conductive substrate thereby apparently reducing the amount of charging and consequently, the sensitivity as a photosensitive element is deteriorated, or since the trapping of negative charges internally present plays a major role, when negative charging and exposure to light are repeated, the resultant accumulation of negative space charge in the photosensitive element increases the residual potential, thus leading to deteriorated reuse properties.
On the other hand, regarding the process of electrophotography and photosensitive elements, U.S. Pat. No. 3,041,167 discloses a process in which an electrostatic latent image is formed through a series of steps comprising a first charging; a light exposure of the entire surface; a second charging in a polarity opposite that of the first charging; and an image-wise exposure to light.
As will be seen in FIG. 2 of the accompanying drawings, the photosensitive element used in this process basically comprises three layers, an electrically insulating surface layer 3, a photoconductive layer 2, and an electrically conductive substrate 1, the electrically insulating surface layer 3 being transparent to radiation to which the photosensitive layer 2 is sensitive.
The process as described above will be briefly explained with reference to FIG. 2. A transparent electrically insulating surface layer 3 in the photosensitive element having the structure as described above is initially charged at a certain polarity (FIG. 2a), and then, entirely exposed to light to form an electric charge opposite to that on the surface at the boundary between the electrically insulating layer 3 and the photoconductive layer 2 (FIG. 2b). Subsequently, a charging in a polarity opposite to the polarity of the first charging is effected to neutralize the charge on the surface, on the one hand, and to induce on the electrically conductive substrate 1 a charge of a polarity opposite to that existing at the boundary as mentioned above, on the other hand. Finally, the element is image-wise exposed to light to discharge the charge existing at the boundary corresponding to the image-wise exposure and ultimately to form an electrostatic latent image at the boundary between the electrically insulating layer 3 and the photoconductive layer 2 (FIG. 2d).
Since treatments such as development, transfer, and cleaning are effected only on the electrically insulating layer, this photosensitive element has various advantages such as extended life and no precautions are required due to possible toxicity problems because the photoconductive layer is covered with the electrically insulating surface layer. One of the features in the process of using this photosensitive element is that the accumulation of space charge is limited because both positive and negative carriers flow through the photosensitive element. However, in the photosensitive element as described above, because charge carriers are injected from the electrically conductive substrate, a good picture image can not be obtained where the sensitivity thereof is deteriorated or the polarity of the charging must be reversed in use because of a decreased S/N ratio (signal/noise ratio).